1. Field of Invention
The present invention pertains to methods and systems for optimizing argon recovery in an air separation unit.
2. Related Art
Cryogenic Air Separation Units (ASUs) have been used to produce oxygen, nitrogen, and argon gases by cooling, liquefying, and distilling air. In a basic system, air is compressed and separated using high- and low-pressure cryogenic distillation columns. In the high-pressure column, nitrogen is separated from the air, creating oxygen rich liquid at the bottom and nitrogen rich liquid and vapor at the top. These products are extracted and some are fed separately to the low-pressure column. Due to the differences in relative volatility between argon, nitrogen, and oxygen, substantially pure gaseous nitrogen forms near the top of the low-pressure column, substantially pure liquid oxygen forms near the bottom of the column, and an argon-rich oxygen gas forms toward the center of the column. The central, argon-rich fraction, called raw argon, is drawn from the low-pressure column and is fed via a stream to an auxiliary, crude argon column. The raw argon stream is rectified into an oxygen rich reflux, which is then sent back to the low-pressure column to be condensed, and crude argon, which can either be sent along as product or further refined.
The raw argon stream, in addition to containing oxygen, typically contains a small amount of nitrogen. The presence of nitrogen creates several processing concerns when recovering argon. To maximize the amount of argon recovered, it is necessary to maximize the amount raw argon extracted from the low-pressure column, and minimize the amount of oxygen extracted in the raw argon stream. Increasing the amount of raw argon drawn from the low-pressure column, however, also increases the amount of nitrogen drawn from the low-pressure column and sent to the crude argon column. If the amount of nitrogen becomes too high, the nitrogen pressure in the top of the crude argon column will have a detrimental effect on the heat transfer ability of the argon condenser, which will negatively affect the flow of gas up the column. Specifically, when the nitrogen concentration in the crude argon column passes a threshold value, the gas flow becomes insufficient to support the liquid hold-up in the column. The liquid falls down the crude argon column and back into the low-pressure column. This is known as “dumping” the crude argon column. The effects of dumping include not only a loss of argon recovery, but also the introduction of significant quantities of liquid into the low-pressure column that contaminate oxygen and nitrogen product purities. Dumping, then, is a costly economic penalty of the operation at high argon recovery rates. To avoid dumping, plants intentionally recover argon at rates significantly below the maximum recovery rate for the plant. Argon is a valuable by-product of the air separation product; consequently, reducing argon column product flow is economically undesirable. In addition, it is desirable to maximize the recovery of argon regardless of the plant mode (i.e., regardless of which product the air separation unit is attempting to produce).
A variety of different approaches are known in the art that attempt to maximize the recovery of argon in an air separation system, while minimizing the risk of dumping. For example, U.S. Pat. No. 4,842,625 to Allam et al. describes a cryogenic air separation process wherein the pressure of the feed gas to the argon column is reduced across a control valve and the argon column is operated at its lowest possible pressure, consistent with a minimum temperature difference across the overhead condenser and the unrestricted return of crude oxygen vapor from the overhead condenser to the low pressure column.
U.S. Pat. Nos. 5,313,800 and 5,448,893 to Howard et al. describes a process for maximizing the recovery of argon wherein a compositional measurement is made of a process variable at one or more preselected stages of rectification, which have been identified as exhibiting high sensitivity to plant process variations. The total nitrogen content in the argon feed is then computed by simulated mathematical correlation from the measurement.
U.S. Pat. No. 5,469,710, also to Howard et al. describes a cryogenic air separation system for improving argon recovery, wherein vapor from the argon column top condenser is turbo-expanded to generate refrigeration and then passed into the lower pressure column.
U.S. Pat. No. 4,784,677 to Al-Chalabi describes a method and apparatus for controlling a process for the separation of air to obtain oxygen, argon, and nitrogen products, and for controlling the composition of the feedstream to a column for producing crude argon. The nitrogen content of the feedstream is directly analyzed in real time and maintained within a preselected range. The results of the analysis are then used to control the operation of the process, for example, by adjusting the reflux or product withdrawal rates.
While each of the systems described above provides certain efficiencies and advantages, there still exists a need to provide an air separation system including a crude argon column that optimizes the recovery of argon while minimizing the chance of dumping.